This invention relates to the subject of electronic emission from the surface of a metal, i.e.--the liberation of electrons from the metal's surface; and more particularly, it relates to the fabrication of cathodes for producing electronic emission. Such cathodes are used, for example, in all vacuum tubes and electron-beam devices.
Basically, in any metal, there exists a large number of valence electrons which are relatively free to move within the bulk of the metal. But at the metal's surface, there is a potential barrier which tends to prevent electrons from leaving the metal. Thus, before an electron can be emitted from a metal, it must possess sufficient energy to overcome the potential barrier. That energy in electron-volts is called the barrier potential.
At a temperature of absolute zero, no electrons have sufficient energy to escape from the metal's bulk; and the largest energy which any electron has is called the Fermi level. Therefore, for electronic emission to occur, an electron must gain an amount of energy which is at least equal to the difference between the barrier potential and the Fermi level. That energy difference is called the work function, and it varies from metal to metal.
Low work function cathodes are, of course, desirable since a lower work function yields a larger current for any given operating temperature, and electric field. And in the prior art, one cathode having a work function of only 2.7 ev is described in U.S. Pat. No. 3,814,975 issued June 4, 1974 to Wolfe et al. That cathode is also illustrated herein in FIG. 1 as indicated by reference numeral 10.
Cathode 10 is comprised of a single crystal tungsten needle 11 whose axis is oriented in the (100) direction, having a ring of zirconium 12 attached to it. (Col. 9, lines 48-52 of U.S. Pat. No. 3,814,975). Suitably, needle 11 is about 30 mil high, about 5 mils across the base, and its tip is rounded with a radius of about 1 um (col. 8, lines 63-68 and col. 9, lines 1-5 of U.S. Pat. No. 3,814,975). Also, a hairpin-shaped filament 13 is attached to needle 11, to provide a means for heating the needle. (Col. 4, lines 48-49 of U.S. Pat. No. 3,814,975).
In operation, the zirconium migrates from the ring 12 over the surface of needle 11. (Col. 9, lines 52-53 of U.S. Pat. No. 3,814,975). This surface migration is indicated by the arrow 14 in FIG. 1. As a result of this surface migration, the zirconium covers the tip of needle 11. There, the zirconium together with oxygen atoms in the residual gas 15 of the vacuum that surrounds needle 11 cause the needle's work function to be reduced from 4.5 ev for pure tungsten to 2.7 ev. (Col. 9, lines 54-60 of U.S. Pat. No. 3,814,975).
An undesirable characteristic, however, of the above described cathode is that its 2.7 ev work function is dependent on the needle's temperature, and on the pressure of the surrounding residual gas 15. (Col. 10, lines 3-6 of U.S. Pat. No. 3,814,975). This characteristic is illustrated herein in FIG. 2. There, beam current I.sub.b along the needle's axis 16 is plotted on the vertical axis, pressure P of the residual gas 15 is plotted on the horizontal axis, and temperature is indicated as T.sub.1, T.sub.2, and T.sub.3. Curves 21, 22, and 23 respectively show how I.sub.b and P are interrelated for temperatures T.sub.1, T.sub.2, and T.sub.3 where T.sub.3 &gt;T.sub.2 &gt;T.sub.1.
The important point here is that given a specific temperature, there is only one optimum operating pressure at which the beam current I.sub.b will peak. At that pressure, the cathode's work function is indeed 2.7 ev. But as the operating conditions vary from optimum, the needle's work function increases, which causes the beam current I.sub.b to drop.
Accordingly, a primary object of the invention is to provide an improved electron-beam cathode.
Another object of the invention is to provide an electron-beam cathode having a work function of 2.7 ev which is substantially insensitive to temperature and pressure fluctuations.
Still another object is to provide a method of making an electron-beam cathode which meets the above objectives.